Steam boilers and generators



June 9, 1964 A. P. PEARCE 3,136,297

STEAM BOILERS AND GENERATORS Filed Oct. 10, 1961 4 Sheets-Sheet l Inventor Arno la( Par'l'e? as 70e a-rQ e il, By

72M Y )7l t'torneys June 9, 1964 A. P. PEARCE STEAM BOILERS AND GENERATORS 4 Sheets-Sheet 2 Filed 0G17. 10, 1961 F/GZ.

Inventor mwL o2 /Qorco as Pea/c e Attorneys June 9, 1964 A. P. PEARCE 3,136,297

STEAM BOILERS AND GENERATORS Filed Oct. 10, 1961 4 Sheets-Sheet 3 `lune 9, 1964 A. P. PEARcE 3,136,297

STEAM BoILERs AND GENERAToRs Filed oct. 1o, 1961 4 sheets-sheet 4 M14/N STEAM OUTLET /ro WAS rE 67 TO FEED TANK 66 FEED TANK.

lnvenlor United States Patent O 3,136,297 STEAM BQILERS AND GENERATORS Arnold Porteous Pearce, 14 Howick Place, Victoria Si., London SW. 1, England, assigner of one-half to Cochran and Company, Annan, Limited, Annan, Dumfriesshire, Scotland Fiied (let. 10, 1961, Ser. No. 144,212 Claims priority, application Great Britain Feb. 28, 1961 6 Ciaims. (Cl. 122-31) This invention relates to steam boilers or generators.

An object of the invention is to obviate or mitigate the formation of scale and/ or the accumulation of solids in the circulatory system of the boiler.

With this object in view the invention provides a steam boiler or generator having primary and secondary superheating and evaporative circuits whereof the former is heated at least mainly by radiation and the latter has a forced circulation and is heated at least mainly by convection, the primary circuitCs) being arranged to receive steam produced in the secondary circuit(s) and to have introduced into such steam feed Water for evaporation thereby, which feed water controls the temperature of the steam leaving the primary circuit(s) and passing to supply.

Small steam boilers or generators designed for compactness and rapid steam raising are subject to failure by over-heating of the tubes due to the deposition of scale. The failure occurs most frequently in the radiant section of the generator Where the heat transfer rate is highest and where, due to this high heat transfer rate,` the formation of scale is most likely to occur.

A further object of the present invention is to obviate or mitigate incidence of scale formation in the primary superheating and evaporative circuit of the boiler or generator, and with this in view the primary circuit is preferably subdivided into a plurality of separate superheating and evaporative sections arranged in series and each having its own supply of water. Means may be provided to withdraw excess water fed to each said section, such withdrawn water carrying with it the salts left behind by that volume of the water which has been evaporated to steam. Part of the water so withdrawn may be returned to feed and part sent to waste, the latter serving as a continuous blowdown, the amount of which will depend on local conditions of water supply.

Embodirnents of the invention will now be described, by way o f example, with reference to the accompanying drawings in which:

FIG. 1 is a side elevation, partly in section, of a first embodiment of the invention;

FIG. 2 shows diagrammatically the arrangement of the primary and secondary circuits;

FIG. 3 shows diagramrnatically a modied form of the circuits of FIG. 2;

FIGS. 4 and 5 are detail views of two different forms of the coils in the furnace chamber of the FIG. 3 arrangement; and

FIG. 6 is a diagrammatic illustration of a further modified steam boiler or generator in accordance with the invention.

Referring firstly to FIGS. 1 to 5. of the drawings, the boiler or generator shown comprises a furnace chamber 1 bounded by a tubular coil 2 constituting the primary circuit and heated by a radiant source, theheating means being a fuel burner 3. The lower end of the coil 2 is connected to a steam outlet pipe 4 through a Vvalve 34, and its upper end is connected by a pipe with the steam space of a combined steam and Water drum 6. A valve 7 for controlled introduction of feed water into theprimary circuit is fitted to the pipe 5 and is arranged to ice be supplied with water by a feed pump 8 via a pipe 9 (see FIG. 2).

The secondary circuit of the boiler comprises tubular coils 10 surrounding the coil 2 and arranged to be heated by the hot gases of combustion passing between the turns of the coil 2. Boiler space 11 housing the coils 10 is connected to a flue or chimney 13. At the lower end the coils 10 are connected through a circulating pump 14 and a pipe 12 with the water space of aforesaid drum 6, and at their upper end they are connected with the steam space of said drum by a pipe 15.

A pipe 16 from the feed pump 8 communicates with the drum 6 through a Water level control valve 17 (see FIG. 2).

Another pipe 18 (FIG. 2) including a circulating valve 19 connects the coils 2 and 10 of the primary and secondary circuits as appears clearly from FIG. 2.

In this figure reference 20 denotes a safety valve in the steam pipe 5, and 21 denotes a blowdown valve for the drum 6.

At the beginning of operating the boiler the valve 19 is open and the valve 34 is closed, so that water is initially circulated through the secondary circuit including the coils 10 and through the primary circuit including the coil 2. During this Vperiod steam pressure is being raised and at the same time coil 2 is flushed to remove any deposits. Before opening valve 34 and supplying steam to the circuit, valve 19 is closed after which steam passes from the steam space of the drum along pipe 5 where it joins a supply of water into coil 2, which supply of water is introduced from pipe 9 via valve 7 in a controlled amount suicient to ensure that the steam leaves the boiler dry or saturated or at any temperature to suit requirements. Point 35 is sensitive to temperature and controls the amount of water passing through valve 7. Steam is, of course, produced by the circulation in the secondary circuit and collects in the upper part of the drum 6 whence it passes through the primary circuit to the supply point or points. Make-up water enters the drum 6 through the water level control valve 17. The valve 19 is used merely as a filling or flushing connection and takes no further part in the operation of the boiler after the initial stages.

From the foregoing it will now be clear that the circulating system is such that the boiler operates in part as a once-through boiler but avoids the trouble that occurs at points in the heating surfaces at which there occur a change of state of the fluid circulated-resulting in the bulk of the solids in the feed water being deposited in the tubes. In the described boiler the primarycircuit takes steam from the secondary circuit and approximately 25% to 30% of the feed water is introduced for evaporation purposes into the steam circulating through the primary circuit. Thus only vapour circulates in the primary circuit and no or substantially no scale is deposited in the tubes thereof.

To ensure that the velocity of steam passing through the primary circuit is sufficiently high to maintain low coil or tube temperatures, the necessary pressure differential is generated in the secondary circuit toprovide for the losses in the piping and de-superheating device.

If desired steam may be bled off the Yprimary circuit at one or more points also provided with water injectorssto control the temperatures at these points. Under a modication the described radiantly-heated steam coil may be arranged to shield a coil in the furnace chamber through which Water isk also Vcirculated for conversion in the main by conducted heat into steam. This arrangement is shown in FIG. 3 where the chain-dotted line indicates the shielded coil 30 connected by lower and upper pipes 31, 32 to the secondary circuit of the boiler. The shielded coil 30 and the steam coil 2 may be disposed closely adjacent and welded together as shown in FIG. 4 or the coil 30 may be disposed within the steam coil 2 and positioned therein by radial spacers 33 as shown in FG. 5. In this modification thewater is introduced for de-superheating at the outletend of the pipe 4, through Valve 36 which is controlled by the temperature at point 37, so that no solids are introduced into the steam coil 2. It will be appreciated that heat transferred from the radiantly-heated steam coil to the shielded water tube is greatly facilitated by conduction of heat through the tube Walls in close contact with one another, so tending to lower the tube Wall temperature of the steam coil 2, which is the primary function of this dual tube construction.

Referring now to FIG. 6 of the drawings, the generator consists of a primary circuit in the form of a number of superheating coils 51 arranged to form the furnace walls of the generator and connected in series into a header 52 which is divided internally by baffles '71 to form separate sections 72.

Into each section 72 of the header other than the first section, water for evaporation is introduced through pipes 53 Where it mixes with the steam from the preceding section 72. At the same time, excess water is collected at each header section 72 and drained off through steam traps 54, passing along pipe 55 which divides into pipes 56 and 57, which go to the feed tank and blowdown.

Valves 58 in pipes 53 are pre-set to control the water supply to each section 72 and valves 73 and 74 in pipes 56 and 57, respectively, control the proportion of the excess Water which is passed to waste. Alternatively, opening of the valves S is controlled in accordance with the steam output of the boiler from a temperature-sensitive point 75 in the steam outlet pipe.

The convection or secondary circuit S9 of the generator, through which the gases of combustion pass after leaving the furnace, is formed of a coil or coils through which Water is circulated by pump 60. The Water is drawn by pump 60 from the lower section of a steam separating vessel 61. After passing through coil 59, the resultant steam and water mixture is returned through pipe 62 to the upper section of the vessel 61 where the steam and Water are separated. The water returns to the lower section of the' vessel 61 for recirculation by pump 60 and the steam passes out of the vessel through pipe 63 to the first Section 72 of the primary or radiant superheating circuit or coil 51.

After passing through the series of radiant superheating -coils 51 the steam is finally passed to process along the steam outlet pipe 76 through stop valve 64.

The feed water for the generator is taken from the feed tank by pump 65 and passed along pipe 66 which divides into pipes 67 and 68. Pipe 68 is connected to the vessel 61 'and the supply of water is controlled by valve 69 to maintain a constant level of water in vessel 61.

The Water supply through pipe 67 to the radiant superheating sections is controlled by valve 70 which is connected directly to the combustion controls so that'at all times the quantity of water has a direct relationship to the steam output of the generator.

Alternatively, should local water conditions permit, pipe 67 could be taken from the discharge of pump 60 as indicated by the dotted line, thus using Water` from the separating vessel 61 for introduction into the radiant superheating coils. The water taken from the header section by the steam traps 54 would therefore serve also as blow-down from the convection or secondary circuit 59.Y

I claim:

1. A steam generator comprising a combustion chamber, Water tubes located in said combustion chamber, a once-through primary superheating circuit disposed in said chamber to be heated predominantly by radiation from said heating means and having at least one iirst tube, at least one secondary circuit comprising a steam and water drum and at least one tube, said tube being disposed in said chamber to be heated predominantly by convection and shielded by said rst tube, means adapted to recirculate fluids in said second circuit through said tube and drum, means connected to both said first and second circuits to receive steam from the latter and transfer same to the former, and means for introducing into said steam in first circuit feed water for evaporation by said steam in quantities to produce by such evaporation, at least 25 0f the total steam output of the generator.

2. A steam boiler or generator as set forth in claim l, said primary circuit beingra first metal coil and said water tubes being a second metal coil closely nested within said first metal coil whereby heat is transferred chiefly by conduction between said first and second metal coils.

3. A steam boiler or generator as set forth in claim 2, including radial spacers interspaced between `said Viirst and second metal coils.

4. A steam boiler or generator comprising heat-radiating means, a primary superheating circuit disposed to be heated predominantly by radiation from said heat-radiating means, a secondary evaporative circuit disposed to be heated predominantly by convection from said heat-radiating means, said secondary evaporative circuit comprising a single tube, means for recirculating fluid Within said single tube, means connected to both said primary superheating circuit and said single tube to receive steam from the latter and transfer same to the former, and means for introducing into said Vsteam when in said primary superheating circuit feed Water for evaporation by said steam and in quantities to produce, by said evaporation, at least twenty-five percent of the total steam output of the boiler or generator.

5. A steam generator comprising a heat radiating means, a primary superheating circuit disposed to be heated predominantly by radiation from said heat-radiating means, a secondary evaporative circuit disposed to be heated predominantly by convection from said heat-radiating means, said secondary circuit comprising a steam and Water drum, means adapted to recirculate uids in said secondary evaporative circuit and said drum, means connected to both said superheating and evaporative circuits to receive steam from the evaporative circuit and transfer same to the superheating circuit, and means for introducing into said steam'when in said primary superheating circuit feed water for evaporation by said steam and in quantities to produce, by said evaporation, at least 25% of the total steam output of the generator.

6. A steam generator comprising a combustion chamber, a fuel burner in said chamber, a once-through primary superheating circuit disposed in said chamber to be heated predominantly by radiation from said fuel burner, a secondary evaporative circuit surrounding said chamber and adapted to be heated predominantly by convection by the hot gases of combustion from said fuel burner, said secondary circuit comprising a steam and water drum, means adapted to recirculate uids through said secondary evaporativel circuit with said drum, means connected to both said superheating and circuits to receive steam from the second thereof and transfer same to the first thereof, and means for introducing into said steam when in said primary superheating circuit feed Water for evaporation by said steam and in quantities to produce, by said evaporation, at least 25% ofthe total steam output of the generator.

References Cited in the file of this patent UNITED STATES PATENTS 1,927,649 Noack Sept. 19, 1933 2,201,618 La Mont May 2l, 1940 2,321,390 vJuzi June 8, 1943 FOREIGN PATENTS 395,189 Great Britain July 13, 1933 742,683 France Jan. 4, 1933 

1. A STEAM GENERATOR COMPRISING A COMBUSTION CHAMBER, WATER TUBES LOCATED IN SAID COMBUSTION CHAMBER, A ONCE-THROUGH PRIMARY SUPERHEATING CIRCUIT DISPOSED IN SAID CHAMBER TO BE HEATED PREDOMINANTLY BY RADIATION FROM SAID HEATING MEANS AND HAVING AT LEAST ONE FIRST TUBE, AT LEAST ONE SECONDARY CIRCUIT COMPRISING A STEAM AND WATER DRUM AND AT LEAST ONE TUBE, SAID TUBE BEING DISPOSED IN SAID CHAMBER TO BE HEATED PREDOMINANTLY BY CONVECTION AND SHIELDED BY SAID FIRST TUBE, MEANS ADAPTED TO RECIRCULATE FLUIDS IN SAID SECOND CIRCUIT THROUGH SAID TUBE AND DRUM, MEANS CONNECTED TO BOTH SAID FIRST AND SECOND CIRCUITS TO RECEIVE STEAM FROM THE LATTER AND TRANSFER SAME TO THE FORMER, AND MEANS FOR INTRODUCING INTO SAID STEAM IN FIRST CIRCUIT FEED WATER FOR EVAPORATION BY SAID STEAM IN QUANTIES TO PRODUCE BY SUCH EVAPORATION, AT LEAST 25% OF THE TOTAL STEAM OUTPUT OF THE GENERATOR. 